(1) Field of the Invention
The present invention relates to an image forming apparatus which forms the image on a recording medium by causing the developer to jump thereto and can be applied to a printer unit in digital copiers and facsimile machines as well as to digital printers, plotters, etc.
(2) Description of the Prior Art
Known methods for forming a visual image on a recording material such as paper etc., in accordance with an electric signal output from computers, word processors, facsimile machines etc., include an ink jet mechanism using ink, thermal transfer process of fusing ink and transferring it, a sublimation process, an electrophotographic process etc.
With recent development to high speed, high quality, and low cost, the ink jet mechanism which has a relatively simple device configuration having non-impact type print heads integrated therein has been often employed. In this ink jet mechanism, however, a good image cannot be obtained due to blooming of liquid ink used, and further, when color printing is performed by superimpositional printing, color mixing from the mixing of inks cannot be obtained. For these reasons, when high quality is needed, printing by the electrophotographic process using toner has been employed.
Prints using toner are free from blooming, and can provide visually excellent images having high color density. Further, when a plurality of colors are mixed for development into color images, these colors can be mixed during the fixing process to provide the correct color mixture. For these reasons, there has been proposed a direct printing mechanism using toners, which performs printing by combination of the simple ink jet process and the toner image.
For example, Japanese Patent Application Laid-Open Hei 6 No. 328,769, has disclosed a means of forming a toner image on paper, which includes a toner flow controlling means having apertures for controlling the flow of toner, a toner supplying means for supplying toner to the toner flow controlling means and a rearside electrode which is disposed on the side opposite the toner supplying means with respect to the toner flow controlling means, and is configured in such a manner that voltages are applied to the toner-flow controlling means in accordance with the image information so as to control the passage of toner through the apertures of the toner flow controlling means, to thereby form a toner image on the paper which is being conveyed over the rearside electrode.
Here, in the above disclosure, i.e., Japanese Patent Application Laid-Open Hei 6 No. 328,769, the toner flow controlling means has a control voltage applying circuit connected thereto, which is composed of an image signal outputting circuit and piezoelectric transformers. This piezoelectric transformer boosts the image signal of relatively low in voltage, into a high voltage, to control the flow of toner.
For example, information-related apparatuses such as printers etc., tend to markedly reduce in size and cost, and hence components constituting such an apparatus are designed as to be reduced in size and cost. Of these, there is a demand that power source circuit be further reduced in size and cost. The power source circuit, however, is composed of a relatively large number of components, and each component needs an anti-heat configuration, thus making it difficult to reduce its size. In particular, the transformer is an essential component in the power source circuit, and wire-coil type transformers have been typically used. However, awire-coil type transformer suffers from various defects such that it is hard to be downsized because of its structure, and the transformer itself is heavy, further still, the conversion efficiency is not very high.
As a means for solving such problems, a piezoelectric transformer has recently drawn attention which transforms input electric energy into mechanical energy by utilizing the piezoelectric effect, and again transforms it into electric energy by utilizing the same effect to thereby boost the voltage. Since the piezoelectric transformer is not a means which performs voltage transformation via magnetic energy using wound wires as in the wire-coil type transformer, it can be designed to be made smaller in size and thickness. If a wire-coil type transformer is attempted to be made thinner, the conversion efficiency would be lowered. Piezoelectric transformers are free from this problem and can achieve a high conversion efficiency.
So far, the means for applying high voltages to the control electrode as the means for controlling the toner flow, has employed high withstand voltage switching elements in order to perform switching of high voltages. The aforementioned disclosure proposed a method which uses piezoelectric transformers, to eliminate the necessity of the high withstand voltage switching elements.
This piezoelectric transformer, however, has a drawback that the output current is low. In a case, for example, where the control electrode is controlled by a single piezoelectric transformer, if the whole of the control electrode is simultaneously turned on, the output current from the voltage source, i.e., the piezoelectric transformer, is markedly increased, so that current exceeding the maximum rated value of the piezoelectric transformer is needed. More specifically, a control electrode to be used for 300 dpi (dot per inch) has 2,560 apertures for A4 sized paper. When one site of the control electrode is turned on, the instantaneous current flowing through the electrode at least needs some 10 .mu.A. When this is simply multiplied by 2,560, the consumption current instantaneously flowing through them amounts to some 100 mA. A typical piezoelectric transformer has a maximum rated current of 10 to 20 mA, hence only a single transformer cannot meet the requirements.
Recently, one which can supply about 100 mA has been developed for special use. This, however, cannot be used, because the safety standard for information processing devices (including OA apparatus) laid by the UL (Underwriters Laboratories Inc.) specifies that the maximum allowable current of the limiting current circuit should be 70 mA at its peak value. Therefore, a piezoelectric transformer of the aforementioned special type cannot be built in to an image forming apparatus. However, use of a multiple number of normal piezoelectric transformers sharply increases the cost, resulting in a disadvantage.